Dispersion of entangled carbon nanotube by melt extrusion · Dispersion of entangled carbon nanotube by melt extrusion Korea-Australia Rheology Journal June 2010 Vol. 22, No. 2 91
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Korea-Australia Rheology Journal June 2010 Vol. 22, No. 2 89
Korea-Australia Rheology JournalVol. 22, No. 2, June 2010 pp. 89-94
Dispersion of entangled carbon nanotube by melt extrusion
Joo Seok Oh1, Kyung Hyun Ahn
1 and Joung Sook Hong
2,*1School of Chemical and Biological Engineering, Seoul National University
2Department of Chemical and Environmental Engineering, Soongsil University(Received January 29, 2010; final revision received February 18, 2010; accepted March 3, 2010)
Abstract
This paper investigates the dispersion of carbon nanotube (CNT) in a polymer melt during iterative extrusionby measuring electrical and rheological properties. 2 wt% CNT as received was mixed with polymer (lowdensity polyethylene) through twin screw extruder. The extrusion was iteratively performed at the same pro-cess condition. At the same time, the rheological and electrical properties were measured. We expected themixing energy applied on entangled CNT increases with process time, which improves the CNT dispersion.The electrical property of intermediate composite was effectively improved by iterative extrusion. After thefifth extrusion, CNT/LDPE composite reached to the conductive electric level (surface resistance ≤E+5 Ω/sq). Also, the rheological properties of composite were increased according to CNT dispersion. Especiallythe rheological properties over lower frequency region were significantly increased by the dispersed nan-otube. This paper suggests that the dispersion by only iterative mixing process results in a disentanglementof CNT and they forms an electrically useful structure. The rheological and electrical measurements indicatethat the CNT disentangled by the iterative mixing method forms a percolation structure.
Keywords : carbon nanotube, melt extrusion, surface resistance, rheology, dispersion
1. Introduction
CNT has been regarded as attractive as a reinforcing
agent by its mechanical strength and good conductivity.
CNT with no defect has a high mechanical strength as
much as 1000 GPa which is comparable to that of metal,
glass fiber, and graphite fiber. In addition, the electrical
properties of CNT can be used to induce conductivity to
non-conducting materials(Chen et al. 2006; Harris 1999;
Lau and Hui 2002). As an electrical filler in polymer, car-
bon black has been used in industry due to its low cost and
easy process. Practically, it needs to be contained over
25 wt% to realize the conductivity in an insulating poly-
mer, which makes much difficult to maintain the mechan-
ical property of polymer by high filler content. Also they
come out from the fractured surface during usage as well.
Instead of carbon black, CNT has been studied because
CNT can provide an electrical property with its low con-
tent by its high aspect ratio and high electrical prop-
erty(Harris 1999). However, previous studies on mechanical
and electrical performance in several CNT applications did
not obtain a consistency in properties of CNT-composite
because of the CNT dispersion is much different from the
dispersion of spherical particles such as carbon black. A
nanoscale diameter of CNT and high aspect ratio gives
much larger surface area, which induces significant entan-
glement between tubes even from synthesis step by a van
der Waals force. Then a disentanglement of entangled CNT
must be worked out to realize its high potential in practical
applications.
To improve the dispersion of CNT, a number of methods
have been introduced to produce CNT/polymer composites
by using a chemical modification of incorporated CNT
(Chen et al. 2006; Harris 1999; Lau and Hui 2002; Krause
et al. 2009; Bauhofer and Kovacs 2009; Li and Shimizu
2007; Chen et al. 2007; Zhu et al. 2003; Lee et al. 2008),
in-situ polymerization(Kumar et al. 2002; Deng et al.
2002; Funck and Kaminsky 2007), surfactant-assisted dis-
persion(Vigolo et al. 2002; Rastogi et al. 2008; Huang et
al. 2009), interfacial polymerization(Haggenmueller et al.
2006), electro-spinning(Wang et al. 2005), and solution
mixing(Qian et al. 2000). Those methods provide suc-
cessful progress in performance with the incorporation of
small amount of CNT (<1% CNT). For example, solution
mixing of CNT and molten polystyrene(PS) led to 30~40%
increase in elastic stiffness and 25% increase in tensile
strength with 1% CNT addition(Qian et al. 2000). But
those dispersion methods demand a usage of solvent and
are not practically applicable due to the formation of severe
aggregation of nanotubes as the content of CNT increases.
All methods are based on the chemical treatment to modify
surface functionality or to remove the impurity of CNT.
*Corresponding author: polymer@ssu.ac.kr© 2010 by The Korean Society of Rheology
Joo Seok Oh, Kyung Hyun Ahn and Joung Sook Hong
90 Korea-Australia Rheology Journal
Without any chemical modification on CNT, the direct
mixing of CNT and matrix did not result in any progress
in the CNT dispersion so far.
In this study, we investigated the dispersion of CNT by
means of polymer melt compounding. Polymer melt com-
pounding with no CNT-chemical treatment is useful, espe-
cially in industry because it does not demand additional
process step. The melt compounding studied so far (Chen
et al. 2006; Harris 1999; Lau and Hui 2002; Krause et al.
2009; Bauhofer and Kovacs 2009; Li and Shimizu 2007;
Chen et al 2007; Zhu et al. 2003) has been performed in
micro-compounder of lab-scale, but we increased the scale
of compounding extruder (L/D~38.7). Furthermore, the
composites were subjected to the extrusion iteratively.
Then, the applied mixing energy on CNT dispersion would
be further increased. The rheological and electrical prop-
erties of polymer composite were measured after each
extrusion. Electrical, rheological and morphological
approaches have shown how CNT dispersion is accom-
plished depending on iterative process.
2. Experimental
2.1. MaterialsLow density polyethylene (LDPE 5322) used in this
study was provided by Hanwha Co. Ltd (Korea). The den-
sity of LDPE is 0.921 g/cm3(ASTM D1505) and it has a
melt flow index of 3.2 g/10 min (ASTM D1238, 190oC/
2.16 kg). CNT used in this study is multiwalled carbon
nanotube purchased from Nanocyl Co. Ltd (NC7000). It is
produced by the chemical vapor decomposition method
and the purity is higher than 87.5%. Fig. 2 presents the
SEM image of CNT powders (NC7000).
2.2. Preparation of CNT/LDPE compositesCNT/LDPE composites were prepared by melt com-
pounding of LDPE and CNT. Melt compounding was per-
formed by using a twin screw extruder (APV Co. D=19 mm,
L/D=38.7) with two kneading zones. Extruder has seven
heating zones and its temperature profile was set to 170,
180, 190, 200, 210, 210, 210oC from feeding zone to die,
respectively. The rotation speed of screw was fixed to
200 rpm. The composition of CNT was fixed to 2 wt% in
this study. As shown in Fig. 1, extrusion was repetitively
performed under the same extrusion condition. After each
extrusion, the extrudate was pelletized and dry-mixed by
mild-tumbler. To investigate the effect of extrusion on
CNT dispersion, the extrudates were then compression-
molded for rheological and electrical measurement using a
Fig. 1. Schematic diagram of iterative mixing process: extrusion-
pelletizing-dry mixing. The CNT/LDPE extrudate is pel-
letized and dry-mixed. Mixing process is repetitively per-
formed at the same process condition. As the iteration
goes on, the entangled CNT disperse more, which
increase rheological property of composite and decreases
electrical resistance.
Fig. 2. SEM picture of CNT powder (Nanocyl NC7000).
Fig. 3. Comparison of rheological properties(complex viscosity
η*, storage modulus G’) of LDPE depending on the num-
ber of extrusion.
Dispersion of entangled carbon nanotube by melt extrusion
Korea-Australia Rheology Journal June 2010 Vol. 22, No. 2 91
hot press molder (PHI Pasadena Hydraulics, Inc.). It was
molded at 190oC for 10 min and then annealed at room
temperature. Because the iterative extrusion of polymer may
cause degradation by shear and heat, the rheological prop-
erties of LDPE were also checked out as shown in Fig. 3.
The rheological properties of LDPE maintains constant for
fifth extrusion, which means there is no change in molec-
ular structure by iterative extrusion.
2.3. CharacterizationThe composite morphology was examined by scanning
electron microscopy (SEM) using a JEOL model Hitachi-
S4800 apparatus operating at an accelerating voltage of
7 kV to 20 kV. The samples for SEM were fractured in liq-
uid nitrogen after each extrusion and then sputtered with
palladium to avoid charging on the fractured surface.
The thermal analysis was measured by differential scan-
ning calorimeter(DSC) using a DSC Q2000 (TA instru-
ments). DSC was performed with sample of about 2 to
5.0 mg between 20 to 150oC by 10oC/min.
The rheological properties were measured at 190oC by
means of RDSII (Rheometrics Inc.) with a parallel plate fix-
ture (25 mm diameter). The complex viscosity (η*(Pas)),
storage modulus (G’(Pa)), and loss modulus (G”(Pa)) were
measured as a function of frequency(ω(rad/s)) using the
dynamic oscillatory mode. As an electrical property, the
surface resistance was measured by Hiresta-Up (Mitsubishi
Chemical Co.) with bar-type probe.
3. Results
The addition of 2 wt% CNT to polymer melt leads to a
significant increase in viscosity by a physical networking
of the dispersed CNTs (Huang et al. 2006). However, this
increase in viscosity of composite is able to be expected
when 2wt% CNT is dispersed sufficiently. From a hydro-
dynamical point of view to accomplish the CNT disper-
sion, the composite needs to be under process for several
times under simple shear flow to apply more energy
against whole van der Waals force between entangled
CNTs (Huang et al. 2006). In fact, it is difficult to expect
the dispersion of CNT by simple extrusion of short pro-
cessing time. If the extrusion time is longer by iterative
extrusion, the CNT dispersion is able to be expected as
shown in Fig. 1.
First of all, 2 wt% CNT/LDPE composite and LDPE
were checked their degradation by iterative extrusion based
on rheology and thermal analysis (DSC). From Fig. 3 and
Fig. 4, it is known that iterative extrusion of short pro-
cessing time does not modify LDPE and composite. Fig. 4
compares the thermal analysis of composite and LDPE
with the number of extrusion. As expected from Fig. 3, the
thermal analysis of LDPE is not changed during repetitive
extrusion. In the case of 2 wt%CNT/LDPE, the slope of
beginning stage of endotherm peak looks changed a little
by iterative extrusion but the maximum peaks are the same
at 108oC, which means the molecular weight of PE is not
changed during iterative extrusion.
Fig. 5 represents the variation of rheological properties
of 2 wt% CNT/LDPE composite depending on the number
of extrusion. Because LDPE was confirmed its consis-
tency of rheological properties over repetitive extrusions
as shown in Fig. 3, it can be considered the increment of
rheological properties is purely caused by the dispersion
of CNT. The storage modulus reflects the structural
change of composite by the influence of relaxation time
Fig. 4. Comparison of thermal analysis (DSC) of LDPE,
2 wt%CNT/LDPE depending on the number of extrusion.
For the DSC measurement, heat scan was performed
twice between 20oC and 150oC by 10oC/min. The DSC
data shown above is on second round.
Fig. 5. Comparison of rheological properties(complex viscosity
η*, storage modulus G’) of 2 wt%CNT/LDPE depending
on the number of extrusion.
Joo Seok Oh, Kyung Hyun Ahn and Joung Sook Hong
92 Korea-Australia Rheology Journal
( , here λ is relaxation time and
H(λ) is relaxation time spectrum)(Larson 1999). The stor-
age modulus of the first extrudate simply increases all over
the observation frequency region compared to that of
LDPE, which is expected by the filling effect in composite.
However, when extruded three times, the storage modulus
increases while the slope at lower frequency region starts
to decrease. Even after the fifth extrusion, the storage mod-
ulus of composite increases further and they present the
plateau at lower frequency region. We believe that this pla-
teau of storage modulus is originated from the network
structure of the disentangled CNTs (Huang et al. 2006).
The disentangled CNT initially increases the relaxation
times like an ultra molecule, which is reflected in the
increases in storage modulus at low frequency region (Lar-
son 1999). Table 1 lists the slope of G’ at lower frequency
depending on the number of iteration. The slope of 2 wt%
CNT/LDPE shows non-terminal behavior as the number of
extrusion increases. The slope of composite after fifth
extrusion significantly decreases from 17.82 to 4.6. As
CNT disentangles further over several extrusions, they
form a physical network structure. Then, it dominates the
rheological behavior of the composite, as can be seen with
the plateau at low frequency region. Meanwhile, the struc-
tural change of CNT/LDPE composite is confirmed
through the surface resistance. Fig. 6 shows the surface
resistance of extrudates with extrusion time. The surface
resistance is simply decreased over the extrusion time,
which means the number of CNT interconnection effec-
tively increases as the extrusion goes on. The surface resis-
tance is much lower when CNT in polymer forms an
electric path homogeneously. With the fifth extrusion, the
composite reaches to the conducting electric level (surface
resistance ≤E+5Ω/sq). Above fifth extrusion of compos-
ite, the decrease of surface resistance becomes slow.
Fig. 7 compares the morphology of CNT/LDPE com-
posite. The morphology of the first extrudate is not so dif-
ferent from that of composite after the fifth extrusion. Over
iterative extrusions, the change of rheological and elec-
trical properties of the composite can be strong evidence to
the change of the CNT dispersion, while the morphological
observations are difficult to discriminate the effect of mul-
tiple extrusions on CNT dispersion.
To ensure those observations, the iterative extrusions were
performed with other commercial multi-walled CNTs (JEIO
(purity 94.1%), CM95 (purity, 95%)). All CNTs show the
same effect of multiple extrusions on rheological properties
and surface resistance of CNT composite. Table 2 lists the
surface resistance and storage modulus of composite of dif-
ferent CNTs depending on extrusion time. The storage
modulus at frequency 0.1 rad/s increases as much as 300%
G′ ω( )H λ( )ω2λ2
1 ω2λ2+
------------------------ λln( )d∞–
∞
∫=
Table 1. Slope of G’ of 2 wt%CNT/LDPE
MaterialsSlope of G’/G’
ω=0.1 at
lower frequency
LDPE 17.8
2 wt%CNT/LDPE(# iteration=1) 14.9
2 wt%CNT/LDPE(# iteration=3) 8.6
2 wt%CNT/LDPE(# iteration=5) 4.6
Fig. 7. Comparison of morphologies of 2 wt%CNT/LDPE after
the first (a-c) and the fifth extrusion (d-f).
Fig. 6. Variation of surface resistance of 2 wt%CNT/LDPE
depending on the number of extrusion.
Dispersion of entangled carbon nanotube by melt extrusion
Korea-Australia Rheology Journal June 2010 Vol. 22, No. 2 93
in case of NC7000. Also, the surface resistance decreases
significantly with extrusion time. Those observations show
that the change of electrical and rheological property of
composite depending on the number of extrusion is not so
different with CNT though CNT has different morphology
or geometry according to the synthesis of CNT. Rather the
dispersion of CNT influences the property of composite
more significantly.
4. Discussion
Based on the observations in this study, it can be known
that the iterative extrusion of CNT composite results in the
increase in storage modulus and viscosity and the improve-
ment of electric conductivity by the enhanced CNT dis-
persion. We believe that the repetitive hybrid mixing of dry
mixing and melt compounding improves effectively the
dispersion of CNT. After extrusion, CNT composite extru-
date solidifies under air and it is continuously pelletized as
small as 3~4 mm of diameter, and then it is dry-mixed
again. The pelletizing and dry-mixing enhance the long
range dispersion of CNT particles as shown in Fig. 8. The
following melt compounding induces the long and short
range dispersion of CNT. To obtain CNT dispersion espe-
cially under viscous medium, it requires large energy
enough to unravel the individual CNT out of the entangled
CNT clumps because CNT has a significant van der Waals
force by large surface area. As can be known from Fig. 3,
LDPE is viscous as much as 100 to 1000 Pas at high shear.
CNT with the aspect ratio of 1000 will take a long time to
diffuse under the given flow condition. In fact, the resi-
dence time of extrusion at 200 rpm (apparent shear rate at
extruder clearance of 2 mm gap>10000 s-1) ranges only 2
to 5 minutes. Then, it is difficult to expect the complete
disentanglement of CNT clumps in a single extrusion
though the complex flow in an extruder is able to induce
dispersion effectively. Therefore, the iterative process of
dry-mixing and melt-compounding effectively induces the
long and short range dispersion of CNT and the property of
the composite changes significantly. Fig. 9 presents the
surface resistance and rheological property of CNT com-
posite over iterative extrusion. The surface resistance lin-
early decreases with iteration. After the fifth iteration, CNT
forms an electrical percolation over observation area and
Table 2. Complex viscosity, storage modulus and surface resis-
tance of composite with different CNT
CNT#
iteration
Viscosity
(Pas) at
shear rate
0.1 rad/s
Storage
Modulus
(Pa) at
shear rate
0.1 rad/s
Surface
resistance
(Ω/sq)
NC7000
1 6.6 K 0.1 K 1.0E+12
3 10.5 K 0.4 K 2.7E+08
5 20.6 K 1.4 K 4.3E+05
JEIO
1 6.6 K 0.1 K 1.0E+11
3 9.6 K 0.3 K 6.3E+09
5 15.5 K 0.8 K 2.2E+05
CM95
1 5.9 K 0.1 K 1.0E+12
3 7.7 K 0.2 K 1.0E+12
5 9.3 K 0.3 K 1.0E+10
Fig. 8. Schematic diagram of CNT dispersion during mixing:
long range dispersion occurs during dry mixing and extru-
sion, short range dispersion is an individual CNT dif-
fusion. A long- and short range dispersion simultaneously
occurs during extrusion and a dry mixing helps long range
dispersion of CNT.
Fig. 9. Variation of surface resistance and complex viscosity of
2 wt%CNT/LDPE depending on the number of extrusion.
Joo Seok Oh, Kyung Hyun Ahn and Joung Sook Hong
94 Korea-Australia Rheology Journal
the surface resistance almost reaches to a saturation value.
However, the rheological properties of the composite
change continuously over extrusions. Especially, the rheo-
logical properties at low shear rate increase significantly,
while those at higher shear rate increase a little. It means
that CNT maintains the diffusion over multiple extrusions
and the multiple extrusions enhance the dispersion con-
tinuously. The dispersed CNTs increase the complex vis-
cosity of CNT composite further. Then, we believe that the
rheological properties indicated the degree of CNT dis-
persion more precisely compared to the electrical property.
5. Conclusions
The iterative mixing process of dry mixing and melt
compounding was found to enhance CNT dispersion. Iter-
ative extrusion was found to apply more mixing energy on
CNT particles to induce CNT dispersion. During iterative
extrusions, the surface resistance of composite decreased
and their rheological properties were increased. The reduc-
tion of surface resistance and the increases in rheological
properties were caused by the disentangled CNT. This
paper shows that the intensive mixing process can induce
a disentanglement of CNT dispersion effectively.
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